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Subject[RFC][PATCH 10/13] mm/vmscan: add helper for querying ability to age anonymous pages
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From: Dave Hansen <dave.hansen@linux.intel.com>

Anonymous pages are kept on their own LRU(s). These lists could theoretically
always be scanned and maintained. But, without swap, there is currently
nothing the kernel can *do* with the results of a scanned, sorted LRU for
anonymous pages.

A check for '!total_swap_pages' currently serves as a valid check as to
whether anonymous LRUs should be maintained. However, another method will
be added shortly: page demotion.

Abstract out the 'total_swap_pages' checks into a helper, give it a
logically significant name, and check for the possibility of page
demotion.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: osalvador <osalvador@suse.de>
---

b/mm/vmscan.c | 28 +++++++++++++++++++++++++---
1 file changed, 25 insertions(+), 3 deletions(-)

diff -puN mm/vmscan.c~mm-vmscan-anon-can-be-aged mm/vmscan.c
--- a/mm/vmscan.c~mm-vmscan-anon-can-be-aged 2021-01-25 16:23:17.044866690 -0800
+++ b/mm/vmscan.c 2021-01-25 16:23:17.053866690 -0800
@@ -2508,6 +2508,26 @@ out:
}
}

+/*
+ * Anonymous LRU management is a waste if there is
+ * ultimately no way to reclaim the memory.
+ */
+bool anon_should_be_aged(struct lruvec *lruvec)
+{
+ struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+
+ /* Aging the anon LRU is valuable if swap is present: */
+ if (total_swap_pages > 0)
+ return true;
+
+ /* Also valuable if anon pages can be demoted: */
+ if (next_demotion_node(pgdat->node_id) >= 0)
+ return true;
+
+ /* No way to reclaim anon pages. Should not age anon LRUs: */
+ return false;
+}
+
static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
{
unsigned long nr[NR_LRU_LISTS];
@@ -2617,7 +2637,8 @@ static void shrink_lruvec(struct lruvec
* Even if we did not try to evict anon pages at all, we want to
* rebalance the anon lru active/inactive ratio.
*/
- if (total_swap_pages && inactive_is_low(lruvec, LRU_INACTIVE_ANON))
+ if (anon_should_be_aged(lruvec) &&
+ inactive_is_low(lruvec, LRU_INACTIVE_ANON))
shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
sc, LRU_ACTIVE_ANON);
}
@@ -3446,10 +3467,11 @@ static void age_active_anon(struct pglis
struct mem_cgroup *memcg;
struct lruvec *lruvec;

- if (!total_swap_pages)
+ lruvec = mem_cgroup_lruvec(NULL, pgdat);
+
+ if (!anon_should_be_aged(lruvec))
return;

- lruvec = mem_cgroup_lruvec(NULL, pgdat);
if (!inactive_is_low(lruvec, LRU_INACTIVE_ANON))
return;

_
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